1. Field of the Invention
The present invention relates to a transmission power control system, and in particular, to a transmission power control system in a mobile phone.
2. Background of the Related Art
In the beginning of the mobile communication technology, a wireless communication system directly processed radio signals using a transmitter and receiver. Today, most of a mobile communication transmission in air communication networks such as a phone net is performed by wire with a wireless system only used for transmissions in an area near a mobile station. A plurality of base stations are required for reciprocal radio communication or the like with the mobile station.
Accordingly, the mobile station only needs to contact the base station for communication. Thus, the mobile station needs to stay within the communication service area of at least one base station.
The communication service area of a base station is called a cell. Each mobile station registers its location to a network for designating one or a small number of cells possibly containing the mobile station when the base station or network wants to call the mobile station. The location registration is closely related to the network service area providing the mobile communication service.
A mobile station can be installed in a ship, an aircraft, a car or a portable mobile station i.e., a mobile phone. Carried by a user, the portable mobile station has to be lightweight. The development of electronics miniaturization has contributed to many lightweight parts. However, batteries have experienced a retarded improvement of power capacity per unit weight. Further, transmitting power has very significant impact to the service area of a cell.
The wider the cell becomes, the larger the size of a battery becomes. Accordingly, the cell has to be reduced in size (e.g., range) to reduce the battery requirements. However, reducing the cell size increases the number of cells and increases the maintenance cost for the network, in particular, in high demand or dense base stations areas.
To reduce battery consumption for the mobile station, the transmitting power from the mobile station to the base station is set according to the distance between the mobile station and the base station. Thus, the mobile station will not transmit more power than the maximum necessary. Accordingly, the magnitude of the power transmitted from the mobile station is fixed to the maximum range of the cell. However, when the distance between the base and mobile station is small (i.e., less than the maximum range), the unnecessary battery consumption is inevitable because the power transmission is more than is necessary. Thus, it is important for the mobile station to generate an exact magnitude of the transmission power by evaluating the distance between the mobile and base station.
FIG. 1 shows a block diagram of a related art transmission part and a transmission power controlling part in a mobile phone. The transmission part is composed of an automatic gain control amplifier 10, a mixer 12, a pre-amplifier 14 and a power amplifier 16.
A high frequency signal from a modulator is inputted to the automatic gain control amplifier 10. The gain of the automatic gain control amplifier 10 is controlled by a gain controlling signal outputted from a mobile station modem 24. A high frequency signal outputted from the automated gain control amplifier 10 is mixed with a local oscillation signal LO in the mixer 12. During the mixing process, a carrier frequency corresponding to the high frequency signal is adjusted upwardly to a frequency of the local oscillation signal LO to because a RF (Radio Frequency) signal.
Not large enough to be an input signal for the power amplifier 16, the RF signal from the mixer 12 is amplified in the pre-amplifier 14. The pre-amplifier 14 amplifies the RF signal to the required magnitude to drive the power amplifier 16 and then outputs the RF signal to the power amplifier 16.
The power amplifier 16 amplifies the RF signal to the magnitude for actual transmission through an antenna 18. A power detector 20 detects a magnitude of the RF signal by converting the transmission power of the actual transmission into a DC signal.
In the power amplifier 16 in the transmission part, the gain is controlled by the mobile station modem 24. An A/D converter (ADC) 22 converts the DC signal from the power detector 20 into a digital signal, which is input to the mobile station modem 24.
The mobile station modem 24 includes various kinds of devices for controlling the mobile station. A variety of data necessary to control the mobile station are stored in a look-up table 25 in the mobile station modem 24. The mobile station modem 24 is supplied with the distance between the mobile and base station through an extra receiving part.
The base station provides the mobile station with the information of the distance by an open-loop power check. The mobile station modem 24 picks up the data for controlling (e.g., a gain controlling signal) the magnitude of the transmission power from the look-up table 25 in accordance with the distance previously supplied from the base station.
A gain controlling signal of the power amplifier 16 among the data picked up from the look-up table 25 is converted into an analog signal by a D/A converter 26 and inputted into a comparator 28. The comparator 28 outputs a signal proportional to the difference between a detected signal of the power detector 20 and an output signal of the D/A converter 26 to the power amplifier 16. The gain of the power amplifier 16 is controlled by the signal from the comparator 28. Further, the output signal of a D/A converter 30 controls the automatic gain control amplifier 10.
The D/A converter 30 outputs a DC signal that has been converted from a Pulse Density Modulation (PDM) signal in the mobile station modem 24. The value of the PDM signal is decided by the open-loop power check between the mobile station and base station with information regarding the transmission power by evaluating both the magnitudes of the transmission power of the mobile station and of the base station when transmitting between the mobile and base station.
The mobile station generates a controlling value programmed in accordance with the relationship of transmission power to distance. The distance is provided to the mobile station from the base station.
The variable range of the gain is narrow in the related art transmission power controlling part of the mobile phone because the power amplifier 16 amplifies the signal, which was already amplified by the pre-amplifier 14.
As described above, the related art mobile phone has various disadvantages. Once the signal outputted from the automatic gain control amplifier 10 is changed whereby the gain of the automated gain control amplifier 10 varies, the final level of the output becomes unstable. Hence, the magnitude of the transmission power has decreased reliability and the operation of the automatic gain control amplifier 10 driven by the programmed data is unreliable.